Preformed crucible



12 1965 P. BURTENSHAW ETAL PREFORMED CRUCIBLE Fid June 8, 1962 United States Patent O 3,16 5,572 r PREFORMED CRUCIBLE V Philip Burteshaw, Watford, and Joseph E. Moore, South Ruislip, England, assigors, by mesne assignments, to National Research Corporation, Camhridge, Mass., a corporation of Massachusetts i Filed June 3, 1962,Ser. No. 201,154

`7 Claims. (Cl. 13-27) In induotion melting furnaces, the metal is melted in a crucible which is supported by a thermally insulatng powder within the induction coil. For the larger sizes of furnace, the crucible may be formed directly from the insulatng powder, thepowder being rammed around a former and sintered.

The insulatng powder is normally contained by the induction coil and its electrical insulation and, when it is necessary to change the crucible either through crucible failure or the necessity of changing the charge material, it is necessary to knock outthe old assembly to ,make the crucible. This may require the removal of the induction coil from* the furnace assembly and is always a lengthy job. In addition, if the furnace is designed to operate under vacuum, all the new material has to be de-` gassed before an effective vacuum can be obtained.

A crucible which can be made as an assembly separate from the coil and easily removable from the coil structure has several advantages. The replaceable assembly may be prepared in advance and at any convenient place, i.e. away from the melting shop floor. The furnace need only be shut down for the time required to remove a crucible assembly and replace it.

In the case of vacuum furnaces, the assembly may be stored in an oven so` that all moisture is removed before the crucible is placed into the furnace tank.

Since the coil is not removed from the furnace, there is no difliculty of alignment of bearings and power feeds for the `pivotable furnace tank.

After the furnace is shut-down, the hot crucible assembly may be removed and stored in a safe place. It is not necessary to leave cooling water circulating to protect the coil.

Replaceable crucible assemblies have hitherto been in the form of two concentric crucibles, the space between the two being filled with the thermal insulation. The outer cylinder or sheath has usually been made from silica, but this material is subject to heat shock and cracking occurs, allowing the powder to leak and binding the assembly within the coil. To overcome this effect and to allow for uneven expansions, silica segments have been suggested, but silica is a very difiicult material to use from the manufacturing aspect.

An object of the present invention is to provide a replaceable crucible assembly which is easily manufactured,

and which overcomes or reduces the difficulties hitherto met with in such assemblies.

From one aspect, the invention provides a replaceable crucible assembly comprisng an outer sheath of nonmagnetic metal of high resistivity, split longitudinally to prevent flow of induced currents, and top and bottom plates of insulatng material, one at least of these plates being removable to allow packing of thermally insulatng material within the sheath, either around an inner crucible or to form a rammed lining.

From another aspect, a replaceable crucible assembly according to the invention comprises a non-magnetic metal sheath of high resistivity split longitudinally by at least one split which is maintained by insulatng material spanning the gap, a crucible mounted within this outer sheath, a top plate of insulatng material Secured to the outer sheath and extending outwardly to provide a support for the assembly, a bottom plate of insulatng mau terial "closing the bottom of the outer sheath, andthermal insulation* packed between the outer sheath and the crucible 'and support-ing the 'crucible from the bottom plate, one of the'said plates being removable' to perrnit packing of the thermally insulatng material around the crucible.

A shaped insulatng brick may be provided at the bottom of the assembly, 'supported by the bottom plate, to provide additional insulation.

The assembly may be supported within the coil of an induction furnace by clipping the top plate to the coil supports and can readily be removed or replaced without disturbance to the coil.

Other parts of the invention are cmbodied in the preferred form which will now be described in some deta'l by way of example with reference to the accompanying drawings the single figure of which is an elevation, partly in section of a crucible arrangement mounted inside a furnace.

In this form, the crucible 2 stands on a base brick 3 which is supported by a laminated glass fibre disc 10. The crucible is surrounded by a metal cylinder 6, the space between the sheath and the crucible being filled with thermally insulatng powder 1.

The cylindrical sheath 6 is made from thin gauge stainless steel which is non-magnetic and possesses high resistivity and is split lengthwise to prevent the flow of induced current. The split 16 is maintained by a mica strip 12 riveted along both edges of the split. The thickness of the cylinder s chosen so as to reduce as far as possible the power induced directly into it.

The cylinder 6 terminates substantially at the level of the center lines of the top and bottom turns of the coil 4 of the induction furnace 13 (shown in chain lines) with which the crucible is to be used. From these levels, the cylindrical sheath is extended by a number of cars 14 which secure the cylinder to a top clamping ring 9 and to the disc 10, and ears 14A which support the bottom plate 10 from ring 9 via sheath 6.

To prevent the refractory insulatng powder 1 from escaping through the edges of the can, a crucible bag 5 of silica or glass cloth is provided enclosing the crucible 2 and the base brick 3. The top edges of this bag are Secured between the clamping ring 9 and a top plate 3 of insulatng material.

This top plate 8 is annular and closes the annular space between the outer cylinder and the crucible. It extends beyond the outer cylinder to provide a flange which, when the assembly is in use, rests on the coil supports 15 of the induction furnace and is clipped thereto by a quick release clamp 7 to locate the crucible correctly in relation to the coil. i

Between the top plate 8 and the crucible 2, an annular recess is provided for packing with asbestos string and cement 11.

The crucible assembly can readily be removed as a whole from the furnace without disturbance of the coil. The bottom plate and brick can be removed, permitting removal of the thermally insulatng powder and its replacement by 'fresh powder, rammed in from the bottom, this being the most convenient method of ramming.

The shaped brick and the bottom plate can then be replaced and the crucible assembly is ready for re-insertion in the induction furnace.

It will be understood that the invention is not restricted to the details of the preferred form described by way of example which can be modified without departure from the broad ideas underlying them.

What is claimed is:

1. A preformed, replaceable crucible assembly for induotion furnaces and the like, comprising an inner crucible, an outer sheath made of non-magnetc metal of Patented Jan. 12, 1965' 3 high resistivity `and constructcd to prevent flow of induced currents therein, and top and bottom members of insulating material, one at least of these members being removable to allow packing of thermally insulating material within the sheath around the inner 'crucible, thermal insulating, material packed between the crucible 'and sheath th e` en tire assembly being held together sothat it can besupported and moved as a single unit to and from its operating location in proximity to a fixed furnace heating element.

2. The assernbly of claim 1 including a shaped insulating brick at the' bot tom of the assembly supported by the bottom member.

3. The assembly of claim 1 wherein the top member i adapted torest on the coil supports of an induction furnace and comprising means for securing the top member to the coil supports, the assembly being supported from the top member so that the assembly can be removed or replaced' Without disturbing the coil.

4. The assembly of claim 1 wherein the sheath is made of thin t gauge stainless steel and constructecl with a vertical split spanned by an insulating strip to prevent flow of induced Currents.

5. A replaceable crucible assembly comprising a nonmagnetic metal sheath of high resistivity split longitudinally by at least one split which is maintained by insulating material spanning the gap, a'crucible'mounted within this outer shea 'th, a top plate of insulating material secured to the outer sheath and extending outwardly from the cruoible to a fixed supporting member to provide a support for the assembly, a' bottom plate ofinsulatng material closing the bottom of the assembly and supporting a crucible hearth to provide an essentially closed annular 'space between the crucible, Sheath, hear th and top plate, and therrnally insulating material packed between the outer sheath and the crucible, one of thesaid plates i V 7. The assembly of claim 5 wherein the bottom plate is supported from the top plate Via the sheath.

References cited in the file of this patent v UNITED STATES"PATENTS 2,635,125

U ton h `A r. 14, '1953. 23,289 Beck Feb. 11, 1958 2,969,411 Gibbs Jan. 24, 1961 

1. A PREFORMED, REPLACEABLE CRUCIBLE ASSEMBLY FOR INDUCTION FURNACES AND THE LIKE, COMPRISING AN INNER CRUCIBLE, AN OUTER SHEATH MADE OF NON-MAGNETIC METAL OF HIGH RESISTIVITY AND CONSTRUCTED TO PREVENT FLOW OF INDUCED CURRENTS THEREIN, AND TOP AND BOTTOM MEMBERS OF INSULATING MATERIAL, ONE AT LEAST OF THESE MEMBERS OF REMOVABLE TO ALLOW PACKING OF THERMALLY INSULATING MATERIAL WITHIN THE SHEATH AROUND THE INNER CRUCIBLE, THERMAL INSULATING MATERIAL PACKED BETWEEN THE CRUCIBLE AND SHEATH, THE ENTIRE ASSEMBLY BEING HELD TOGETHER SO THAT IT CAN BE SUPPORTED AND MOVED AS A SINGLE UNIT TO AND 